Polyimide films are well known polymers, and include any such polymers having an -imide linkage in the backbone or in the side chain of the polymer. These materials are commercially available under several different trade names, and are known in the packaging of electronic circuits. These films are valued for their high thermal stability and excellent electrical properties. They have high heat resistance, good dielectric properties, are solvent-resistive, and can be applied by simple processing. While they are often used in molding and composite industries, the electronics industry uses them extensively as thin films.
Although polyimides have many attractive properties, it is very difficult to pattern films of polyimides, because they are not easily etched or dissolved away by the usual techniques. Thus, wet chemical processing is extremely difficult. In the art, reactive ion etching has been known as a technique for patterning polyimide films. However, this is a very slow process in which reactive ions, such as CF.sup.+.sub.3, react with the polyimide.
A representative polyimide is given by the following formula: ##STR1##
All polyimides carry the functionality ##STR2## as an imide group.
K-I Kawakami, et al., U.S. Pat. No. 4,247,496 describes a method for treating a thin surface of a plastic material with an ultraviolet light after which the plastic is stretched. The plastic materials treated are polypropylene or polyethylene terephthalate. The UV light has a wavelength ranging from 180 to 400 nm, and is emitted by sources such as mercury lamps, fluorescent lamps, xenon lamps, and carbonarc lamps. The ultraviolet light treatment causes cracking in a surface layer (5-10 nanometers) of the plastic. These cracks make stretching easier and leave a surface which contains widened cracks therein. There is no photoetching involved in this method because it is important that only a thin surface layer be affected. The patent does not recognize that selected wavelengths of ultraviolet radiation can be used to efficiently photoetch polyimide. In fact, polyimides are not stretchable plastics and do not have linear chains which can be stretched and oriented.
"Effective deep ultraviolet Photoetching of Polymethyl methacrylate By an Excimer Laser" by Y. Kawamura et al., in Appl. Phys. Lett. 40(5) 1 Mar. 1982, pp. 374-375 describes the use of a KrF excimer laser to photoetch a polymethyl methacrylate body. The method described uses a contact mask with a single opening therein.
"Direct Etching of Resists by U.V. Light" by N. Ueno et al., Ja. J of App. Phys. Vol. 20, No. 10, Oct. 1981, pp. L709-712 describes the use of a "D.sub.2 Lamp" to photoetch photo resists such as polymethyl methacrylate, polymethyl isopenyl ketone and modified Novolak resin.
Since thin films of polyimide are extremely useful, it would be a distinct advantage to have a reliable technique for patterning these films. In particular, it is desirable to have a process which will provide high resolution patterning of films of polyimide. The cross-referenced R. Srinivasan patent application describes one such technique where the polyimide is controllably photoetched by far U.V. radiation of wavelengths less than 220 nm, to produce patterns therein. The patent application indicates that the wavelength must be under 220 nm and gives as its only examples two U.V. radiation sources: a 185 nm low pressure Hg lamp and the 193 nm ArF laser. Further, limitations include the requirement for photoetching in an oxygen containing atmosphere and the location of the patterned mask either upon the surface of the polyimide or very closely adjacent thereto. These limitations upon the patterning of polyimide are severe for high volume manufacturing. Particularly limiting is the inability of the prior art to find ways of using the more cost effective U.V. radiation range of between about 240 nm to 400 nm. The depth of the etching is preferrably at least 50 nanometers, and can be entirely through the polyimide film.
It is, therefore, a primary object of the present invention to provide a method for the cost effective direct etching of polyimide.
It is another object of the present invention to provide a technique for etching polyimide films using ultraviolet radiation in the range of 240 nm to 400 nm for photodecomposing the polyimide.
It is a further object of this invention to provide a method for the projection photoetching of a polyimide layer in a desired pattern with the sides of the openings having a positive slope so that a subsequently deposited layer would completely fill the openings.
It is a still further object of this invention to provide a method for etching patterns in polyimide layers carried upon integrated circuit substrates and subsequently depositing an electrical circuit pattern thereover to form electrical interconnections to elements of the integrated circuit.